Superficial and peripheral dose in compensator‐based FFF beam IMRT

Abstract Flattening filter‐free (FFF) beams produce higher dose rates. Combined with compensator‐based intensity modulated radiotherapy (IMRT) techniques, the dose delivery for each beam can be much shorter compared to the flattened beam MLC‐based or flattened beam compensator‐based IMRT. This ‘snap shot’ IMRT delivery is beneficial to patients for tumor motion management. Due to softer energy, superficial doses in FFF beam treatment are usually higher than those from flattened beams. Due to no flattening filter, thus less photon scattering, peripheral doses are usually lower in FFF beam treatment. However, in compensator‐based IMRT using FFF beams, the compensator is in the beam pathway. Does it introduce beam hardening effects and scattering such that the superficial dose is lower and peripheral dose is higher compared to FFF beam MLC‐based IMRT? This study applied Monte Carlo techniques to investigate the superficial and peripheral doses in compensator‐based IMRT using FFF beams and compared it to the MLC‐based IMRT using FFF beams and flattened beams. Besides varying thicknesses of brass slabs to simulate varying thicknesses of compensators, a simple cone‐shaped compensator was simulated to mimic a clinical application. The dose distribution in water phantom by the cone‐shaped compensator was then simulated by multiple MLC‐defined FFF and flattened beams with varying apertures. After normalization to the maximum dose, Dmax, the superficial and peripheral doses were compared between the FFF beam compensator‐based IMRT and FFF/flattened beam MLC‐based IMRT. The superficial dose at the central 0.5 mm depth was about 1% (of Dmax) lower in the compensator‐based 6 MV FFF (6FFF) IMRT compared to the MLC‐based 6FFF IMRT, and about 8% higher than the flattened 6 MV MLC‐based IMRT dose. At 8 cm off‐axis at depth of central maximum dose, dmax, the peripheral dose between the 6FFF and flattened 6 MV MLC demonstrated similar doses, while the compensator dose was about 1% (of Dmax) higher. Compensators reduce the superficial doses slightly compared to open FFF beams, but increases the peripheral doses due to scatter in the compensator.


| INTRODUCTION
Historically, raw x-ray beams produced at the accelerator target were modified by an interceding flattening filter, rendering crossbeam profiles which were fairly flat over the clinically applicable range of depths. This was done primarily for the ease of manual/ forward treatment planning. However, with the advent of inverse planning techniques, the radiation beams are no longer required to be flat, and accelerators with flattening filter-free (FFF) beams became available. The major advantage of the FFF beams is higher dose rates, leading to potentially shorter delivery times. 1,2 Because of the absence of the dominant component responsible for scatter the flattening filterperipheral doses are usually lower with FFF beams. 2,3 Compensator-based intensity modulated radiotherapy (IMRT) has been applied clinically with accelerators that are not equipped with multi-leaf collimators (MLC). 4 The intensity modulation is accomplished by a compensator of varying thicknesses, usually made of brass using a computerized milling machine. The intensity modulation resolution in compensator-based IMRT plans is the same as it is in MLC-based IMRT plans. In treatment planning, an optimized ideal beam fluence map is converted into the two-dimensional compensator thickness matrix for delivery. Even with accelerators equipped with MLC, compensator-based IMRT technique has been used clinically 5 because of shorter delivery times 6,7 and fewer monitor units. 8 Upon combining FFF beams with compensator IMRT techniques, the beam-on time can be reduced compared to the MLC-based or compensator-based IMRT with conventional beams, with each beam only taking a few seconds to deliver, 9 which makes potential delivery of a breath-hold treatment for each IMRT beam feasible. Such a possible type of 'snap shot' IMRT delivery would be beneficial to patients whose tumors show significant respiratory-associated motion. This breath-hold motion management strategy would decrease treatment times for patients who are often elderly and positioned with their arms uncomfortably overhead in a customized full body immobilization device. This very short beam-on time advantage makes breath-hold treatments feasible for the patients who otherwise would not be candidates for this motion management technique.
Brass compensators are commercially available so that clinics do not need to have milling machines to make them. Along with advantages, there are also some disadvantages for both the FFF beams and compensator-based IMRT techniques. The compensators require additional time and cost to be manufactured, require the therapists to enter the room between the fields, and usually have limited modulation range. 6 Due to the lower average energy, superficial doses in FFF beam treatment are usually higher than those from flattened beams. [10][11][12] Superficial dose, defined as dose at shallow depth inside treatment field, is often a concern of patient's skin reaction to radiotherapy. 13 In compensator-based IMRT with FFF beams, a compensator in the beam pathway may serve simultaneously as a beam hardening filter and a scatterer. This complex interaction may affect both the superficial dose 14       Because flat brass slabs were used, the mean energies in the FFF configuration were essentially flat across the field, while the mean energy of the 6 MV flattened beam, as expected, was higher at the central axis.
The beam hardening effect can also be seen in Fig. 6(b), which shows the normalized spectra comparison among the various beams from Fig. 6(a). The spectra were normalized so that the area under each curve (i.e., the total fluence) was the same, set at 100%. After

| DISCUSSION
The main advantage of the compensator-based IMRT using FFF beams would be the shorter per-beam delivery time, enabling the voluntary breath-holding motion management technique feasible for a larger subset of patients. 21 The volumetric modulated arc therapy (VMAT) usually takes a shorter overall treatment time, 22 but each arc is relatively long, making compensator-based IMRT with FFF beams more practical for voluntary breath-hold treatment delivery.
As SBRT treatment strategies are being increasingly used in the treatment of patients with lung, liver, and pancreas cancers, further clinical prospective evaluation of these techniques may be warranted.
It is well known that the lower average energy in an open FFF beam results in a higher superficial dose compared to a similar flattened beam. [10][11][12] The compensator could harden the FFF beam, depending on the thickness. Even with a 0.5 cm thick brass slab, the mean energy of the 6FFF beam changes from 1.17 MeV to 1.31 MeV (Fig. 6). Because of this energy difference, the 0.5 cm brass in the compensator-based IMRT made the superficial dose slightly lower than the FFF MLC-based IMRT (Fig. 3). With a 2 cm brass slab, the mean energy of the 6FFF beam was still lower than that of the flattened beam (Fig. 6). The thickness of material in an area of a clinical compensator projecting on the target is expected to be by far not thick enough to harden the beam to match the flattened beam. Thus, in practice, when a FFF beam with a compensator is used for IMRT, the superficial dose in the treatment area may be higher than when using flattened beam with MLC, and slightly lower than that using FFF beam with MLC.
Based on the simulation results in this study, the peripheral dose was similar between the FFF and flattened beams in MLC-based IMRT.
When combining the FFF beam with a compensator, the slightly higher peripheral dose is attributed to scatter coming from the compensator.
Only the peripheral dose near the field edge (~3 cm out of field) was analyzed in this study.